Antennas for ultra-wideband applications

ABSTRACT

An antenna comprising a radiating element for transmitting and receiving communication signals is disclosed. A load and a feed are connectable to the radiating element and that the feed is spaced apart from the load. The radiating element is a planar loop having two free ends to which the load and the feed are connected. The load has two distal terminals, one of which is connected to one of the two free ends and the other is provided for connecting to one of grounding and another radiating element.

FIELD OF INVENTION

The invention relates generally to antennas. In particular, it relatesto planar antennas for ultra-wideband applications.

BACKGROUND

Ultra-wideband (UWB) radio systems transmit and receive communicationsignals as modulated impulses. The duration of the modulated impulses istypically very short and is of the order of a few fractions of ananosecond (ns). This allows the modulated impulses to have frequencyranges that are extremely broad, typically of a few gigahertz (GHz). Thebroad frequency ranges of the UWB radio systems are therefore distinctlydifferent from conventional narrow-band radio systems. This distinctionof the UWB radio systems require a unique set of regulations implementedby a regulatory body specifically for communication systems that arebased on UWB technology. The regulations limit the radiated power levelsand signal spectra of the UWB radio systems in order to facilitate undueinterference to the conventional narrow-band radio systems which occupya part of the frequency spectrum of the UWB radio systems.

One such regulation, as stipulated by the US Federal CommunicationCommission (FCC), requires that the emission levels and spectra of theradiated pulses of a UWB radio system to have an effective isotropicradiated power (EIRP) below −41.3 dBm/MHz for a 10 dB bandwidth thatcovers a frequency range from 3.1 to 10.6 GHz. This regulation defines aspectral limit mask for all UWB radio systems.

Previous studies have shown that emission and reception patterns of aUWB radio system are significantly affected by its antennacharacteristics. Therefore, the emission and reception patterns of theUWB radio system are typically modified to conform to FCC emissionregulation on the limit mask by appropriately designing the antennacharacteristics.

Besides meeting the limit mask regulation, antennas of a UWB radiosystem should be designed to fulfill a number of requirements. Firstly,the UWB radio system has a bandwidth that is as broad and well-matchedas possible for achieving broadband capability and attaining high systemefficiency. Secondly, operating power of the UWB radio system is as lowas possible for attaining high power efficiency. Thirdly, the UWB radiosystem has a linearised phase transfer response for providing minimalsignal distortion. Finally, the UWB radio system generates radiatedpulses with maximum power in a desired direction.

Numerous attempts have been made to fulfill the requirements throughvarious designs of antennas for the UWB radio system. More notableexamples are transverse electromagnetic mode (TEM) horns andself-supplemental antennas, such as spiral antennas. Both types ofantennas feature very broad and well-matched bandwidths. However, pulsesgenerated by both types of antennas are distorted and suffer fromdispersion due to frequency-dependant changes in their respective phasecenters.

Bi-conical and disk-conical antennas have less distortion and haverelatively stable phase centers for achieving a broad and well-matchedbandwidth. This is because resistive loadings are used to eliminatereflection of radiated pulses occurring at transmission ends of bothantennas. However, both antennas are bulky in size and are thusunsuitable for small and portable UWB devices.

In conjunction with the abovementioned requirements for a UWB radiosystem, another important consideration for designing a UWB antenna isthe preclusion of interference to conventional in-band or out-band radiosystems. The UWB antenna is required to function as an efficientradiator that precludes interference to in-band systems such as W-LANoperating at 5.2 or 5.8 GHz or out-band systems operating at 0.99 to 3.1GHz.

Further attempts have been made to provide UWB antennas with broadbandcapability and compliancy with requirements for non-interference withexisting in-band and out-band radio systems. In U.S. Pat. No. 6,437,756,Schantz teaches a notched planar monopole to attain band-notchedcharacteristics with a well-matched bandwidth for a voltage standingwave ratio (VSWR) of less than 2:1. However, the well-matched bandwidthis not sufficiently broad for UWB applications.

In U.S. patent application 2003/0090436 A1, a shorted planar monopolehaving a shorting pin at the bottom of the monopole is proposed bySchantz et al. for size reduction. However, in order to maintainradiation efficiency, the shorting pin and a feed to the monopole areseparated far apart, thus rendering the lateral size of the monopolelarge. The bandwidth of the monopole is also not broad enough for UWBapplications.

In U.S. patent application 2002/0122010, McCorkle proposes using a smallannular planar monopole to achieve a broad and well-matched bandwidth.However, the annular planar monopole does not exhibit band-notchedcharacteristics for the fulfillment for non-interference with existingin-band and out-band radio systems.

There is therefore a need for an antenna for a UWB radio system which isdimensionally small and for improving system efficiency and reducinginterference with existing radio systems.

SUMMARY

Embodiments of the invention are disclosed hereinafter for UWBapplications having a small dimensional size for improving systemefficiency and for reducing interference with existing radio systems. Inparticular, an electrical load is positioned in proximity to a feed toprovide a bandwidth spectrum with a specified notched band.

In accordance with one aspect of the invention, there is disclosed anantenna for ultra-wideband applications, the antenna comprising aradiating element for transmitting and receiving communication signals.A load and a feed are connectable to the radiating element and that thefeed being spaced apart from the load by a predetermined distance. Theradiating element is a planar loop having two free ends. The load hastwo distal terminals, one of the two distal terminal being connected toone of the two free ends of the planar loop and the other distalterminal of the load and another terminal of the feed are provided forconnecting to one of grounding and another radiating element. The twodistal terminal of the load being spaced apart by a predeterminedseparation.

In accordance with another aspect of the invention, there is disclosed amethod for configuring an antenna for ultra-wideband applications, themethod comprising the steps of providing a radiating element having acenter opening and two free ends. The two free ends are connectable to aload and a feed, wherein the load and the feed each has a terminalconnectable to one of grounding and another radiating element and theradiating element is spatially continuous between the load and the feed.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention are described in detail hereinafter withreference to the drawings, in which:

FIGS. 1A and 1B are schematic views of a monopole and a dipolerespectively according to a first embodiment of the invention havingannular radiating elements;

FIG. 2 is a plot showing impedance matching and transfer functioncharacteristics of the monopole of FIG. 1A;

FIGS. 3A and 3B are schematic views of a monopole and a dipolerespectively according to a second embodiment of the invention havingblock shape radiating elements; and

FIGS. 4A and 4B are schematic views of a monopole and a dipolerespectively according to a third embodiment of the invention havingsemi-annular radiating elements.

DETAILED DESCRIPTION

With reference to the drawings, antennas that are dimensionally smallfor ultra-wideband (UWB) applications according to embodiments of theinvention are disclosed for improving system efficiency and reducinginterference with existing radio systems.

Various conventional methods for designing a UWB antenna have previouslybeen proposed. These conventional methods have limited improvement insystem efficiency or reduction in interference with existing radiosystems. Other conventional methods of designing the UWB antenna suggesta need for large antenna dimensions.

For purposes of brevity and clarity, the description of the invention islimited hereinafter to UWB applications. This however does not precludeembodiments of the invention for other applications that require similaroperating performance as the UWB applications. The functional principlesfundamental to the embodiments of the invention remain the samethroughout the various embodiments.

In the detailed description provided hereinafter and illustrationsprovided in FIGS. 1A to 1B and 3A to 4B of the drawings, like elementsare identified with like reference numerals.

Embodiments of the invention are described in greater detail hereinafterfor an antenna for ultra-wide band (UWB) applications.

FIG. 1A shows the geometry of an antenna 100 according to a firstembodiment of the invention for UWB applications. The antenna 100 is amonopole having a radiating element 102 with a center opening fortransmitting and receiving communication signals to and from anotherantenna. The antenna 100 is preferably planar and fabricatedmonolithically on a substrate, such as a printed circuit board (PCB) oran integrated circuit (IC) chip. The communication signals comprisepulse signals having a bandwidth of a few gigahertz (GHz).

The radiating element 102 is formed in the shape of an annular loop,wherein the annular loop is not closed and has at least two end portions104, 106. The center opening of the radiating element 102 is preferablyannular and concentric with the radiating element 102. Two substantiallyparallel free ends 108, 110 extend from the end portions 104, 106,respectively, of the annular loop away from the center opening of theradiating element 102. The extension for which the two free ends 108,110 extend from the end portions 104, 106 of the annular loop isinversely proportional to the operating frequency of the antenna 100.Specifically, the larger the size of the extension corresponds to alower operating frequency of the antenna 100. The amount of extension ofthe two free ends 108, 110 also affects the impedance matchingcharacteristic of the antenna 100.

The end portions 104, 106 and the two free ends 108, 110 are spacedapart by a first predetermined distance g and maintainedtherethroughout. Given the limitations of controlling dimensions duringthe fabrication of the antenna 100, the first predetermined distance gis variably dependable on a given requirement for impedance matching ofthe antenna 100. In this first embodiment of the invention, the firstpredetermined distance g is preferably but not limited to approximately0.5 mm. The radiating element 102 is dimensionally dependable on aninner radius r₁ and an outer radius r₂ and has a substantially uniformwidth of r₂-r₁ therethroughout the annular loop. The outer radius r₂ ispreferably approximately 7.5 mm. The radiating element 102 is preferablyfabricated with conductive material, for example copper.

An electrical load 112 having a first and second terminal has one of thefirst and second terminal connected to the free end 108 of the radiatingelement 102. The electrical load 112 can be a passive or active elementfor providing a resistive or reactive loading, depending on otherelements used for forming the antenna 100. The other of the first andsecond terminal of the electrical load 112 is connected to ground via aground plane 114. The radiating element 102 is connectable to the groundplane 114 through the electrical load 112 for forming a monopole. Thetransmission and reception functionality of the antenna 100 issubstantially independent of the orientation between the radiatingelement 102 and the ground plane 114. The spacing between the free end108 of the radiating element 102 and the ground plane 114 defines asecond predetermined distance s. The second predetermined distance s isdependable on the dimension of the electrical load 112 and is preferablykept at a minimal. For example, when a shorting load is used, the secondpredetermined distance s is zero. When a lump load, such as a chipresistor is used, second predetermined distance s is dependent on thedimension of the chip resistor.

A feed 116 is connected at one terminal to the free end 110 of theradiating element 102 for transferring of communication signals to theantenna 100. The feed 116 is spaced apart from the load 112 by the firstpredetermined distance g. The feed 116 can be balanced or unbalanced andprovides alternating current to the radiating element 102 for thegeneration of modulated impulses. The other terminal of the feed 116 isconnected to ground via the ground plane 114.

The configuration of the radiating element 102 facilitates theattainment of broadband capabilities, which is dependable on thephysical geometry of the antenna 100. During the operation of theantenna 100, the electrical load 112 and the feed 116 each carries analternating current that is out-of-phase from one another. Superpositionof signal radiation generated from the electrical load 112 and the feed116 causes cancellation of the radiation at a particular frequencyregion of the operating bandwidth of the antenna 100. This is becausethe electrical load 112 and the feed 116 are in proximity to each otherand are carrying out-of-phase alternating currents.

In FIG. 1B, a dipole 1000 of the first embodiment of the invention isformed by connecting another radiating element 118 to the electricalload 112 and feed 116 of the radiating element 102 in place of theground plane 114. The feed 116 preferably has a differential feedingstructure for providing both the radiating elements 102, 108 withcurrents that are substantially similar in magnitude. The otherradiating element 118 is substantially symmetrical to the radiatingelement 102. The dipole 1000 has similar performance characteristics asthe antenna 100.

FIG. 2 is a graph that shows measured and simulated test results of theimpedance matching and transfer function characteristics of the antenna100 of FIG. 1A. An annular antenna (not shown) having the same loopdimensions as the radiating element 102 but without the electrical load112 connected thereto is also measured for comparison purposes. Theimpedance matching and transfer function of the antenna 100 aresimulated and measured over a UWB bandwidth with a frequency range ofapproximately 1 to 12 GHz.

The measured and simulated test results show the antenna 100 having awell-matched impedance matching characteristic throughout the frequencyrange of 1 to 12 GHz.

The transfer function characteristics, more specifically the frequencyresponse, of the antenna 100 and the annular antenna are represented by|S₂₁|. The frequency response of the antenna 100 has a notched band atthe lower frequency range of the UWB bandwidth. This notched band is notapparent for the annular antenna. The notched band facilitates thepreclusion of interference with other existing radio system and ispreferably alterable for specific regulatory requirements. Thealteration is achievable by modifying the physical dimensions such asthe first predetermined distance g of the antenna 100.

In this first embodiment of the invention, the notched band appears neara lower bandwidth edge of approximately 3.1 GHz. The notched band may bealtered to appear in other desired frequency range such as 5 to 6 GHzwhile maintaining the frequency response of the antenna 100 forcomplying with other regulatory requirements. Additionally, thefrequency response of the antenna 100 is modifiable by changing at leastone of the inner radius r₁ and the outer radius r₂.

FIGS. 3A and 3B show a second embodiment of the invention in the form ofa monopole 300 and dipole 3000 respectively. The radiating elements 302,306 in the second embodiment of the invention 300, 3000 have geometriesof a block-shape loop with a block-shape center opening. The radiatingelements 302, 304 perform the same functionality and have similarimpedance matching and transfer function characteristics as the firstembodiment of the invention 100, 1000.

FIGS. 4A and 4B show a third embodiment of the invention in the form ofa monopole 400 and a dipole 4000 respectively, wherein the radiatingelements 402, 404 are semi-annular loops with semi-annular centeropening. Similar to the second embodiment of the invention 300, 3000,the third embodiment of the invention 400, 4000 performs the samefunctionality and has comparable impedance matching and transferfunction characteristics as the first embodiment of the invention 100,1000.

The various embodiments of the invention are suitable for a wide rangeof applications, such as UWB wireless communication systems, portableUWB devices and other consumer electronic systems that require antennasfor UWB applications. The embodiments of the invention may be appliedadvantageously to portable UWB systems that require preclusion ofinterference with other existing communication systems that operates inspecific bandwidths. The small physical dimension of the antenna 100reduces power consumption and has a well-matched broadband capability.Collectively, this results in achieving a UWB radio system having lowerpower consumption, higher system efficiency and compliant to regulatoryrequirements.

In the foregoing manner, an antenna having notch band characteristicsfor UWB applications is disclosed. Although only a number of embodimentsof the invention are disclosed, it becomes apparent to one skilled inthe art in view of this disclosure that numerous changes and/ormodification can be made without departing from the scope and spirit ofthe invention. For example, the radiating elements may be constructedfrom conductive materials in other geometrical forms, such as ellipses,triangles, polygons or annuli. Electrical loads may be implemented usingpassive or active circuit elements in order to attain impedance matchingand the feed may be balanced or unbalanced, depending on the use ofeither a dipole or monopole for antenna implementation.

1. An antenna for ultra-wideband applications, the antenna comprising: aradiating element for transmitting and receiving communication signals;a load connectable to the radiating element, the load having a firstterminal and a second terminal being substantially distal to the firstterminal; and a feed having a terminal connectable to the radiatingelement, the feed being spaced apart from the load by a firstpredetermined distance, wherein the radiating element is a planar loophaving at least two free ends and the load has first and secondterminals, one of the first and second terminals of the load beingconnected to one of the two free ends of the planar loop and the otherof the first and second terminals of the load and another terminal ofthe feed are provided for connecting to one of grounding and anotherradiating element, and the two distal terminals of the load being spacedapart by a second predetermined distance.
 2. The antenna of claim 1,wherein the other of the first and second terminals of the load andanother terminal of the feed are connected to another radiating element.3. The antenna of claim 2, wherein the radiating element and the otherradiating element are substantially symmetrical.
 4. The antenna of claim1, wherein the radiating element is annular.
 5. The antenna of claim 4,wherein the radiating element has a center opening.
 6. The antenna ofclaim 5, wherein the center opening is annular and concentric with theradiating element.
 7. The antenna of claim 1, wherein the radiatingelement is spatially continuous between the load and the feed.
 8. Theantenna of claim 1, wherein the radiating element is laid on asubstrate.
 9. The antenna of claim 1, wherein the load is one ofresistive and reactive.
 10. The antenna of claim 1, wherein the load isone of balanced and unbalanced.
 11. The antenna of claim 1, wherein theantenna is monolithic.
 12. The antenna of claim 1, wherein the frequencyresponse of the antenna is characterised by a band-notch being alterableby dimensions of the radiating element.
 13. The antenna of claim 12,wherein the bandwidth of the frequency response of the antenna ismaintained during formation of the band-notch.
 14. The antenna of claim1, wherein the first predetermined distance is approximately 0.5millimeters.
 15. The antenna of claim 1, wherein the secondpredetermined distance is dependable on the dimensions of the load. 16.A method for configuring an antenna for ultra-wideband applications, themethod comprising the steps of: providing a radiating element having acenter opening and two free ends; providing a load having a terminalconnectable to one of the two free ends; and providing a feed having aterminal connectable to the other of the two free ends; wherein each ofthe load and the feed has another terminal connectable to one ofgrounding and another radiating element and the radiating element isspatially continuous between the load and the feed.
 17. The method ofclaim 16, wherein the radiating element is substantially annular andconnected to ground for forming a monopole.
 18. The method of claim 16,wherein each of the other terminals of the load and feed is connected toanother radiating element for forming a dipole.
 19. The method of claim18, wherein the feed is differential.
 20. The method of claim 16,wherein the radiating element and the another radiating element aresubstantially symmetrical.